The effect of Si on increasing the oxidation resistance was
concentration dependent. With Si content higher than 1.14 wt.%,
the formation of a continuous silicon oxide at the substrate/scale
interface was responsible for the improved oxidation resistance.
However, for steel containing 0.51 wt.% Si (S05), a different scale
microstructurewas observed, as shown in Fig. 8(a). The X-ray mappings for Fe, O, and Si in the oxide layer formed on the S05 specimen
surface oxidized at 900 ◦Cfor 10 hare shownin Fig. 8(b)–(d), respectively.
Silicon oxide bands parallel to the specimen surface were
found dispersed in the iron oxide layer. Banded internal oxide formation
has also been observed in Ag–Mg alloys. The mechanisms
for the formation of banded internal oxide in alloys with noble parent
and active alloying elements (such as Ag–Mg alloys), have been
reported by Douglass et al. [21]. However, banded structure formation
in the external scale, as shown in Fig. 8, has seldom been
discussed. The schematic diagram in Fig. 9 may explain parallel
silicon oxide formation embedded in the iron oxides. For low Sicontaining
steels, an outer iron oxide layer was initially formed.
The less protective iron oxides permit oxygen transport through
this oxide layer and cause oxygen dissolution into the steel substrate
(Fig. 9(a)). As demonstrated in Fig. 9(b), internal oxidation
of active Si elements could take place when oxygen supersaturation
occurs. Internal silicon oxide precipitation could induce a
Si-depleted zone just beneath this internal oxide sublayer. With
prolonged exposure in air at high temperature, oxidation of Fe in
both the internal oxidation zone and the Si-depleted zone continued.
In the case where Si outward diffusion was low in the steel
substrate, oxidation in the Si-depleted zone gave rise to an oxide
layer almost free of silicon oxide, as depicted in Fig. 9(c). Continued
oxygen dissolution and supersaturation in the substrate under the
scale could again cause silicon oxide precipitation and Si-depleted
zone formation. Repeated formation of successive internal oxide
layers and Si-depleted zones followed by subsequent iron oxidation
in both zones could eventually result in a banded microstructure
scale, as depicted in Fig. 9(d).